1. Field of the Invention
The present invention relates to a wavelength division multiplexing filter, a wavelength division multiplexing system, and a wavelength division multiplexing method. More particularly, the present invention relates to a wavelength division multiplexing filter having a variable filter shape, and a system for arbitrarily and efficiently multiplexing optical signals of multiple occupation spectral widths.
2. Description of the Related Art
<Wavelength Grid>
In wavelength multiplexing, individual communication channels are multiplexed being assigned to wavelengths. For such wavelengths, standardized wavelengths called wavelength grids, which are recommended by International Telecommunication Union-Telecommunication Standardization Sector (ITU-T) G.694.1, are used.
Wavelength grids are defined at regular intervals starting at a frequency of 193.100 THz, which is defined as anchor frequency. FIG. 6 is a diagram for illustrating wavelength grids. FIG. 6 shows wavelength grids at intervals of 100 GHz. As shown in FIG. 6, frequencies apart from the anchor frequency by 0, ±100 GHz, ±200 GHz, and so on represent wavelength grids.
The standardization of wavelength grid has brought commonality of specifications of optical components that involve wavelength selection, such as optical filters and light sources, among various manufacturers and provided extremely significant industrial advantages.
<Wavelength Division Multiplexing Filter>
In design of a wavelength (or frequency) division multiplexing system, a bandpass filter for multiplexing or demultiplexing wavelength channels is one of important factors (see Japanese Patent Laid-Open Nos. 2004-297228 and 2006-243571, or U.S. Patent Application Publication 2005/0146655, for example). An ideal wavelength division multiplexing filter is one that passes components within a certain frequency range (or a frequency slot) with low loss and eliminates other frequency components. As shown in FIG. 6, a frequency slot is a frequency band assigned to a channel and is a frequency (or wavelength) range defined by boundaries at midpoints between neighboring wavelength grids. A filter characteristic graph with its horizontal axis representing frequency and vertical axis representing passage loss is called a filter shape.
An ideal wavelength division multiplexing filter would have a rectangular filter shape in which loss is low and flat within a predetermined frequency slot, is infinite outside the slot, and steeply changes at the boundary. FIG. 6 shows a frequency slot in a range of ±50 GHz around 193.3 THz as well as an example of a filter shape for multiplexing or demultiplexing a channel corresponding to the frequency slot.
<Filter Synthesis>
Effect given by a filter on a signal is generally linear and the principle of superposition can be employed. That is to say, a series connection of multiple filters can be handled as one filter. Also, within a range in which effects other than the filter are also linear, filtering effects of those filters when regarded as a single filter do not change even if the order of the filters changes.
<Representative Wavelength Division Multiplexing Filter: Fixed Type>
For wavelength division multiplexing filters currently used in wavelength division multiplexing optical transmission systems, several characteristic filters are selectively used. One of such filters is a fixed filter that multiplexes a number of wavelengths into one signal or conversely demultiplexes such a signal into the individual wavelengths, typified by Arrayed Waveguide Grating (AWG). Since filter characteristics of an AWG is not very steep, a filter called interleaver is sometimes used as a complement thereto (see Japanese Patent Laid-Open No. 2004-297228, for example).
An interleaver has one input and two outputs or the reverse, and selects and demultiplexes frequency slots alternately in terms of frequency. Assuming that wavelength grids are numbered, the interleaver classifies (or couples) signals into an even-number wavelength group and an odd-number wavelength group. The interleaver is connected to an AWG, thereby demultiplexing into each individual wavelengths. The filter shape of an interleaver is more rectangular than that of an AWG.
<A Representative Wavelength Division Multiplexing Filter: Variable Type>
For introduction of a new wavelength path and/or bypassing of a wavelength path that is dynamic to some extent, wavelength division multiplexing filters of variable type are also recently used. Known wavelength division multiplexing filters of variable type include Wavelength Blocker (WB) and Wavelength Selective Switch (WSS). Such variable filters are designed to change their filter shape based on electrical control signals (see Japanese Patent Laid-Open No. 2006-243571, for instance).
FIG. 7 schematically illustrates a representative configuration of a variable filter. As shown in FIG. 7, a variable filter is based on the structure of a spectroscope using a diffraction grating 13, and diffracted beam is guided to a mirror 15 that can vary reflection factor or angle from position to position in accordance with an electrical control signal like pixels of a liquid crystal display. Having one pixel correspond to one frequency slot, it is possible to switch between passing (or “through”) and blocking for each frequency slot. A WSS uses a variable-angle mirror which can electrically change reflection direction and has a structure in which multiple fibers are arranged in an array as input/output fibers 10. By slightly varying reflection angle from one wavelength to another, path selection on a per-frequency-slot basis is possible.
Some of such WBs and WSSs have a characteristic of being able to combine bands of multiple channels to form one bandpass filter (see U.S. Patent Application Publication No. 2005/0146655, for example). Specifically, such a filter is characterized by forming a completely one bandpass filter with almost no boundary between combined frequency slots. FIG. 8 shows filter shapes obtained by varying the filter shape of a 50 GHz-interval WB in three ways. FIG. 8 plots the filter shapes with a small vertical shift in order to clearly show the individual filter shapes.
FIG. 8 shows the ranges of three frequency slots. Filter shape A is a filter shape that is obtained by setting only slots 1 and 3 to “through” and other slot to “blocking”. Similarly, filter shape B is a filter shape obtained by setting only slots 1 and 2 to “through” and other slot to “blocking”, and filter shape C is a filter shape obtained by setting all the slots to “through”. As shown in FIG. 8, when channels that are set to “through” neighbor each other, boundaries between them are smoothly connected and form no depression. Such variable filters are now generally available. Since almost all channels are used being set to “through” at individual nodes in an actual optical network that includes Optical Add/Drop Multiplex (OADM) and/or Wavelength Cross Connect (WXC), use of a filter having such characteristics can reduce signal spectrum narrowing effect caused by a filter and associated degradation of transmission performance.
<Asymmetry Interleaver>
When the transmission rate differs from channel to channel, the modulation spectral width also differs, and thus a required bandwidth also differs from channel to channel. However, when channels of various transmission rates exist together, frequency slots are secured to suit a channel that requires the widest bandwidth because wavelength grids of a wavelength division multiplexing system are basically at regular intervals. This has two problems. One problem is that utilization of finite and precious transmission bandwidths lowers and is uneconomical because a wide bandwidth is automatically allocated even to a channel that does not require much bandwidth. Another problem is the necessity to decide a channel that requires the widest bandwidth at the time of system introduction.
Even if a wide bandwidth that was not originally envisaged becomes necessary per channel or conversely a prepared wide bandwidth ends up being unnecessary as a result of changes in future technical trends, it is difficult to change filters in a system that has already been put into operation. This is because if filters are to be changed, currently used wavelength channels would all have to be temporarily evacuated to another system.
If it were possible to vary frequency slot width from channel to channel, a signal of a narrow spectrum could be accommodated into a narrow frequency slot and a signal of a wide spectrum into a wide frequency slot, which could improve efficiency of accommodation for the entire wavelength multiplexing. One technique devised for this purpose is an asymmetry interleaver (see “Optical add/drop multiplexer with asymmetric bandwidth allocation and dispersion compensation hybrid 10-Gb/s and 40-Gb/s DWDM transmission”, Fishman, D. A. et. al., OFC/NFOEC 2006, OWI64, 2006, and “10G/40G-Hybrid Dense-WDM Systems with Flexible OADM Upgradability”, K. Nakamura et. al., ECOC2003, Tu3.6.6, 2003, for example). A typical interleaver is designed such that a port for inputting/outputting even-number wavelengths and a port for inputting/outputting odd-number wavelengths have the same filter shape, whereas the asymmetry interleaver has this ratio be displaced from 50% by intention, e.g., 67%, 33% . . . And a signal of a wide spectral width uses a wavelength grid on the side of wider frequency slots and a signal of a narrow spectral width uses a frequency grid on the side of narrower frequency slots.
When a wavelength division multiplexing optical transmission system handles optical signals of different spectral widths, if frequency slots are fixed and at regular intervals, there would be uneconomically a lot of gaps when the slots accommodate signals of narrow spectral widths, or conversely, they cannot accept signals of wide spectral widths. To resolve this inconvenience, a method of alternately arranging narrow and wide frequency slots has been devised, but as its way of allocation is fixed and not flexible, still leaving the problem of bandwidth waste such as when a system is not operated as originally planned.
Thus, wavelength division multiplexing filters of variable type (WB and WSS) are used, but they have resolution limits. To allow free modification of filter shape, a very fine resolution is required but this is impractical in terms of technology and cost. Thus, it is required to limit the resolution to the minimum amount required, but in turn gaps have to put between channels, which again poses the problem of diseconomy.
This problem is explained using FIG. 13. As a specific example, suppose that a signal having a spectral width that approximately fits into a frequency slot of 100 GHz width (hereinafter called 100 GHz-wide signal) and a signal having a spectral width that approximately fits into a frequency slot of 50 GHz width (hereinafter called 50 GHz-wide signal) are mixed. It seems that the 100 GHz-wide signal can also be handled by using a filter for the finer 50 GHz interval as a wavelength division multiplexing filter, but it is impossible and this will be described below.
Wavelength grids are a series that always includes the original wavelength grids even when the frequency interval between them is reduced to a half or a quarter. This is because when the frequency interval is halved, a new wavelength grid is added at the midpoint between neighboring original wavelength grids. Therefore, a WB or WSS designed for wavelength grids of a half or quarter frequency interval cannot be used for the original wavelength grids. Hereinafter, wavelength grids of such an arrangement standardized by ITU-T G.694.1 will be called standard wavelength grids.
Referring to FIG. 13, it is understood that if a WB or WSS for 50 GHz interval is to be used for multiplexing or demultiplexing of the 100 GHz-wide signal whose center frequency is on standard wavelength grids of 100 GHz interval, its filter shape would overflow into the neighboring channels or an available bandwidth is halved. It can be also seen that use of a finer WB or WSS for 25 GHz interval does alleviate but cannot solve these problems. To alleviate these problems by increasing resolution to the point where there is substantially no trouble, resolution has to be considerably enhanced, which is difficult in terms of cost and technology.
As outlined above, use of a variable filter can allow mixed arrangement of optical signals having different spectral widths with flexibility of a certain degree, but it has the problems of diseconomy associated with unusable and wasted bandwidth formed between wavelength grids and/or reduction of available bandwidths in each frequency slot.